1. Field of the Invention
The present invention relates to various nanowire structures, and to interconnected nanowire networks comprising such structures. The present invention also relates to the use of these nanowire structures and interconnected nanowire networks in fuel cells.
2. Background of the Invention
Nanomaterials, and in particular, nanowires have the potential to facilitate a whole new generation of electronic devices. For example, In certain cases, uses of nanomaterials have been proposed that exploit the unique and interesting properties of these materials more as a bulk material than as individual elements requiring individual assembly. For example, Duan et al., Nature 425:274-278 (September 2003), describes a nanowire based transistor for use in large area electronic substrates, such as, displays, antennas, and the like that employ a bulk processed, oriented semiconductor nanowire film or layer in place of a rigid semiconductor wafer. The result is an electronic substrate that performs on par with a single crystal wafer substrate that can be manufactured using conventional and less expensive processes than those used to manufacture poorer performing amorphous semiconductors, which is also more amenable to varied architectures, such as, flexible and/or shaped materials.
Graphene layers, a single atom layer of carbon formed as a sheet, have found applications in electrochemical cells and as flexible, strong substrates and coatings. See e.g., U.S. Pat. Nos. 5,677,082, 6,303,266 and 6,479,030. In addition, rolled or folded layers of graphene can form carbon nanotubes and the like, see e.g., U.S. Pat. Nos. 6,582,673, 6,749,827 and 6,756,026.
Fuel cells are devices that convert the chemical energy of fuels, such as hydrogen and methanol, directly into electrical energy. The basic physical structure or building block of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side. A schematic representation of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell is shown in FIG. 6. In a typical fuel cell as shown in FIG. 6, a fuel (e.g., methanol or hydrogen) is fed to an anode catalyst that converts the fuel molecules into protons (and carbon dioxide for methanol fuel cells), which pass through the proton exchange membrane to the cathode side of the cell. At the cathode catalyst, the protons (e.g., hydrogen atoms without an electron) react with the oxygen ions to form water. By connecting a conductive wire from the anode to the cathode side, the electrons stripped from fuel, hydrogen or methanol on the anode side, can travel to the cathode side and combine with oxygen to form oxygen ions, thus producing electricity. Fuel cells operating by electrochemical oxidation of hydrogen or methanol fuels at the anode and reduction of oxygen at the cathode are attractive power sources because of their high conversion efficiencies, low pollution, lightweight, and high energy density.
For example, in direct methanol fuel cells (DMFCs), the liquid methanol (CH3OH) is oxidized in the presence of water at the anode generating CO2, hydrogen ions and the electrons that travel through the external circuit as the electric output of the fuel cell. The hydrogen ions travel through the electrolyte and react with oxygen from the air and the electrons from the external circuit to form water at the anode completing the circuit.Anode Reaction: CH3OH+H2O→CO2+6H++6e−Cathode Reaction: 3/2O2+6H++6e−→3H2OOverall Cell Reaction: CH3OH+3/2O2→CO2+2H2O
Initially developed in the early 1990s, DMFCs were not embraced because of their low efficiency and power density, as well as other problems. Improvements in catalysts and other recent developments have increased power density 20-fold and the efficiency may eventually reach 40%. These cells have been tested in a temperature range from about 50° C.-120° C. This low operating temperature and no requirement for a fuel reformer make the DMFC an excellent candidate for very small to mid-sized applications, such as cellular phones, laptops, cameras and other consumer products, up to automobile power plants. One of the drawbacks of the DMFC is that the low-temperature oxidation of methanol to hydrogen ions and carbon dioxide requires a more active catalyst, which typically means a larger quantity of expensive platinum (and/or ruthenium) catalyst is required.
A DMFC typically requires the use of ruthenium (Ru) as a catalyst component because of its high carbon monoxide (CO) tolerance and reactivity. Ru disassociates water to create an oxygenated species that facilitates the oxygenation of CO, which is produced from the methanol, to CO2. Some existing DMFCs use nanometer-sized bimetallic Pt:Ru particles as the electro-oxidation catalyst because of the high surface area to volume ratio of the particles. The Pt/Ru nanoparticles are typically provided on a carbon support (e.g., carbon black, fullerene soot, or desulfurized carbon black) to yield a packed particle composite catalyst structure. Most commonly used techniques for creating the Pt:Ru carbon packed particle composite are the impregnation of a carbon support in a solution containing platinum and ruthenium chlorides followed by thermal reduction
A multi-phase interface or contact is established among the fuel cell reactants, electrolyte, active Pt:Ru nanoparticles, and carbon support in the region of the porous electrode. The nature of this interface plays a critical role in the electrochemical performance of the fuel cell. It is known that only a portion of catalyst particle sites in packed particle composites are utilized because other sites are either not accessible to the reactants, or not connected to the carbon support network (electron path) and/or electrolyte (proton path). In fact, current packed particle composites only utilize about 20 to 30% of the catalyst particles. Thus, most DMFCs which utilize packed particle composite structures are highly inefficient.
In addition, connectivity to the anode and/or cathode is currently limited in current packed particle composite structures due to poor contacts between particles and/or tortuous diffusion paths for fuel cell reactants between densely packed particles. Increasing the density of the electrolyte or support matrix increases connectivity, but also decreases methanol diffusion to the catalytic site. Thus, a delicate balance must be maintained among the electrode, electrolyte, and gaseous phases in the porous electrode structure in order to maximize the efficiency of fuel cell operation at a reasonable cost. Much of the recent effort in the development of fuel cell technology has been devoted to reducing the thickness of cell components while refining and improving the electrode structure and the electrolyte phase, with the aim of obtaining a higher and more stable electrochemical performance while lowering cost. In order to develop commercially viable DMFCs, the electrocatalytic activity of the catalyst must be improved.
A structure combining nanowires, for example semiconductor nanowires, and graphene layers has not been heretofore been disclosed. In addition, nanowires comprised of carbon, or covered with a carbon-based layer, have also not heretofore been disclosed. The nanowire-based structures and networks disclosed herein possess unique properties and characteristics that allow their use in various applications from substrates and supports to membranes and filtration. The present invention also provides nanowire composite membrane electrode catalyst support assemblies comprising the various structures described throughout that provide a highly porous material with a high surface area, a high structural stability and a continuum structure. The composite structures are provided as a highly interconnected nanowire supported catalyst structure interpenetrated with an electrolyte network to maximize catalyst utilization, catalyst accessibility, and electrical and ionic connectivity to thereby improve the overall efficiency of fuel cells, at lower cost, etc.